A method for preparing an object to be tested, having a given relative permittivity, intended to be illuminated by an incident electromagnetic wave. The method includes: providing a part including a cavity for housing the object and at least one extension element made from a material having a relative permittivity that is preferably equal to that of the object, the extension element at least partially delimiting the cavity and extending to either side of the cavity in a passage direction of the cavity, over a length at least equal, on either side of the cavity, to one third of the length of the cavity in the passage direction, and placing the object in the cavity, such that the object is in contact with the extension element in the passage direction.